1. Field of the Invention
This invention relates to a stirring device for automatically measuring dissolved oxygen.
2. Description of the Prior Art
Dissolved oxygen automatic measuring instruments are used for continuously measuring the amount of dissolved oxygen in waste water in municipal water systems, industrial plants and sewage treatment facilities. Such measuring instruments are specified in JIS K0803, and consist of a detector portion and an indicator-recorder portion, comprising an electrode, an electrode holder, a transducer and an indicator. An electrode is immersed in a water sample in which dissolved oxygen is to be measured, but in the case of diaphragm type electrodes, the concentration of oxygen adjacent to the electrode diaphragm is reduced to indicate lower-than-actual values if the water sample is stagnant or if the flow speed thereof is low, so that the sampled water must be given a flow speed of 20 cm/sec or more in order to give an accurate reading. Bubbles, etc. separated from water before or during measurement may adhere to the electrode diaphragm, causing errors in measurement.
In order to obtain accurate and consistent measurement of dissolved oxygen in water, the flow velocity of water contacting the electrode diaphragm must be high and steady, e.g., about 20 cm/sec. Means for stirring the water around the bottom end of an electrode have been used heretofore. Stirring means of the prior art were disclosed in Japanese Utility Model Laid-open Nos. 130258/83 and 130260/83. The former has an electromagnet disposed below the sensing portion of a sensor (the bottom face of an electrode), the electromagnet being energized by an on-off control, and the water is stirred by a horn- or spiral-shaped blade intermittently reciprocated in the direction approximately perpendicular to the axis of the electrode, i.e., horizontally, by the electromagnet. The latter has a stirring body with a brush implanted thereon in place of the blade in the former which moves in the same way as the former.
In addition to the stirring means of the prior art described above, means such as shown in FIG. 5 are also used. In this figure an electrode 11 is inserted in a vessel containing a water sample W in which dissolved oxygen is to be measured and a stirrer 16 having N and S poles is disposed at the bottom of the vessel 12. Underneath the vessel 12 is disposed a rotatable magnet 15 having N and S poles, the stirrer body 16 being rotated by rotating the magnet 15.
In the stirring means of the prior art described above, since the stirrer body is reciprocated or rotated horizontally near the lower end of the electrode, the water movement is not sufficient to generate a fast and steady flow velocity of water contacting the diaphragm so that the output of the electrode is unstable, and bubbles adhering especially to the central portion of the electrode are not removed sufficiently. A motor and a rotor body may be installed on the electrode itself to stir the water, but this adds to the weight of the electrode and possibly causes bending in the long shaft connecting the motor at the top and the stirrer at the bottom of the electrode. Also in a method in which a stirrer bar is inserted in a BOD bin for measurement, a steady flow velocity cannot be obtained, resulting in the problem of inconsistent data.